
 // Definition for a binary tree node.
  struct TreeNode {
      int val;
      TreeNode *left;
      TreeNode *right;
      TreeNode() : val(0), left(nullptr), right(nullptr) {}
      TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
      TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  };
 
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        return dfs(root, 0);
    }

    int dfs(TreeNode* root, int presum)
    {
        presum = presum * 10 + root->val;
        if (root->left == nullptr && root->right == nullptr)
            return presum;
        int ret = 0;
        if (root->left) ret += dfs(root->left, presum);
        if (root->right) ret += dfs(root->right, presum);

        return ret;
    }
};